In this century, the shortage of fresh water is expected to surpass the shortage of energy as a global concern for humanity, and these two challenges are inexorably linked. Fresh water is one of the most fundamental needs of humans and other organisms. Each human needs to consume a minimum of about two liters per day in addition to greater fresh-water demands from farming as well as from industrial processes. Meanwhile, techniques for transporting fresh water or for producing fresh water via desalination tend to be highly demanding of increasingly scarce supplies of affordable energy.
The hazards posed by insufficient water supplies are particularly acute. A shortage of fresh water may lead to famine, disease, death, forced mass migration, cross-region conflict/war (from Darfur to the American southwest), and collapsed ecosystems. In spite of the criticality of the need for fresh water and the profound consequences of shortages, supplies of fresh water are particularly constrained. 97.5% of the water on Earth is salty, and about 70% of the remainder is locked up as ice (mostly in ice caps and glaciers), leaving only 0.75% of all water on Earth as available fresh water.
Moreover, that 0.75% of available fresh water is not evenly distributed. For example, heavily populated countries, such as India and China, have many regions that are subject to scarce supplies. Further still, the supply of fresh water is often seasonally inconsistent. Typically confined to regional drainage basins, water is heavy and its transport is expensive and energy-intensive.
Meanwhile, demands for fresh water are tightening across the globe. Reservoirs are drying up; aquifers are falling; rivers are dying; and glaciers and ice caps are retracting. Rising populations increase demand, as do shifts in farming and increased industrialization. Climate change poses even more threats in many regions. Consequently, the number of people facing water shortages is increasing.
Massive amounts of energy are typically needed to produce fresh water from seawater (or to a lesser degree, from brackish water), especially for remote locations. Reverse osmosis (RO) is currently the leading desalination technology. In large-scale RO plants, the energy/volume required can be as low as 4 kWh/m3 at 30% recovery, compared to the theoretical minimum around 1 kWh/m3, although smaller-scale RO systems (e.g., aboard ships) are less efficient. Another popular method is the multi-stage flash (MSF) distillation, also an energy and capital intensive process.
Rather than extracting pure water, electrochemical methods, such as electrodialysis (ED) and capacitive desalination (CD), extract just enough salt to achieve potable water (<10 mM). Current large-scale electrochemical desalination systems are less efficient than RO plants at desalinating seawater (e.g., 7 kWh/m3 is the state of the art in ED), but become more efficient for brackish water (e.g., CD can achieve 0.6 kWh/m3). In general, existing techniques for removing salt from water, some of which have existed for centuries, tend to be expensive or complicated or both.
Energy and water are becoming more and more intertwined, especially in the context of oil and gas production. Large amounts of water are typically required to extract oil and gas, and millions of gallons of high salinity [i.e., a high total-dissolved-solids (TDS) content] water are produced. This produced water is unfit for industrial or domestic use and unsafe to discharge into the rivers, ground, or any other sink whereby it may mix with the public water supply. Furthermore, most of this water is also unfit for recycling in oil and gas extraction. Often, extraction sites are located far from fresh water sources making it expensive to transport the water and imperative to recycle the water produced on site. For example, extraction of shale gas and oil by hydraulic fracturing requires 5-7 million gallons of water per well. 20-40% of this volume flows back to the surface as highly saline water. This water needs to be treated before it can be recycled or discharged. The higher salinity of flowback and produced water (up to 8 times more saline than seawater) renders most existing desalination processes insufficient. Higher TDS accelerates membrane fouling and quickly increases energy consumption. Evaporative processes suffer with scaling and low recovery rates. In light of these developments, it is even more important to implement new desalination processes that can overcome these challenges.